In England/Wales law a contract consist simply of offer and acceptance. This can be verbal, but if

    (a) it wasn't witnessed by an independent person (i.e. it can't be proved)
or (b) one or both parties to the contract weren't competent to enter into the contract

then it definitely isn't worth the paper it's written on.

If an agreement is tentative then it isn't a contract and, since the musician was asking for confirmation, then that suggests it was a tentative agreement.

HOWEVER

It might be better anyway to think less of the legal implications and more about trying to steer a right course by which everyone is as satisfied as possible.

Your friend may need to do one or more or all of the following.

    (a) CERTAINLY apologise that the person making the original agreement was not competent to do so, not having sought agreement from the committee.
    (b) In the circumstances, make sure the booking is clearly cancelled.
    (c) If the musician is worth booking anyway, sort out another possible date.

In any case, make sure that everything is done in a friendly way. Having it put about that you don't honour your agreements isn't going to do the club any good.

:-)
Ian